The present invention pertains to magnetic sensors. Particularly, the invention pertains to magnetoresistor sensors utilized for sensing magnetic fields or disturbances of magnetic fields. More particularly, the invention pertains to magnetically closed-loop, switched-magnetization magnetic sensors.
Magnetic-field sensors have application in magnetic compassing. They also have been a means for detecting magnetic-field variations caused by components in machinery such as metal rods, gears, cams, vehicles, the earth's magnetic fields, explosive mines, weapons, and minerals in mines, among other things.
One kind of magnetic sensor is the magnetoresistive sensor which uses a device composed of magnetoresistive material whose resistance changes in the presence of a magnetic field. Many materials exhibit some magnetoresistance. The effect is particularly large in ferromagnetic materials. However, even aluminum has a magnetoresistance. A very effective magnetoresistive material is a nickel-iron alloy which may be referred to as permalloy.
Any specimen of ferromagnetic material has a magnetization, or a magnetic moment per unit volume--a vector quantity defined at each point in the material. A bar magnet, for example, has a net volume magnetization because the majority of the magnetic moments in the bar are aligned parallel to one another. Suppose one has a very long, thin film of ferromagnetic material such as permalloy, and a current is running along the length of the film. The magnetization of the film generally forms an angle with the current and the film's resistance depends on this angle. When the magnetization is parallel to the current, the resistance of the film is at a maximum, and when the magnetization is perpendicular to the current, the resistance is at a minimum, or nominal, value. Thus, if a permalloy film is subject to an external magnetic field, the field acts on the magnetization, rotating it and thereby changing the film's resistance. It is this change of resistance that is noted and utilized in the measurement of magnetic field variation.
Magnetoresistive sensors are capable of measuring magnetic fields as small as 0.00001 gauss and fields as large as 100 gauss. To illustrate the significance of these magnitudes, a medium-sized car 10 feet away from a sensor would produce a perturbation in the earth's field of about 0.01 gauss; the fields in the vicinity of a magnetic tape are about 10 gauss and the field at the tip of a bar magnet is about 1000 gauss. Magnetoresistive sensors have a large frequency range in that they can measure direct current fields or varying fields having frequencies in excess of 1 gigahertz.
The drawback of commonly used permalloy magnetoresistors is temperature sensitivity which is typically 3000 parts per million per degree Centigrade. Thus, manufacturers have constructed Wheatstone bridges, each having four magnetoresistors arranged in such a fashion as to cancel this first-order temperature effect. However, because of variations in magnetoresistors such as thin-film permalloy resistors caused by nonuniform deposition, the four magnetoresistors are not matched sufficiently to cancel out all of the imbalances in the bridge. When biased with a voltage or current, the bridge will exhibit a temperature-dependent offset in the output. Such output is unacceptable for many applications of magnetic field sensing that require accuracy over a temperature range. It has been suggested in the related art that a magnetoresistive device may be operated as a closed loop sensor in order to reduce cross-axis sensitivity and nonlinearity of the output from the bridge. In such a configuration, the bridge output is magnetically nulled by a feedback coil and functions at the same point on the transducer input-output curve which makes the output linear versus the input field. Further, since the macromagnetization direction is held at a fixed position, the cross-axis sensitivity to external fields is minimized. The closed-loop configuration applied to the magnetoresistive bridge has the advantages of high linearity and dynamic range; but unfortunately, this closed-loop configuration provides no greater reduction in temperature sensitivity than the magnetoresistive bridge in an open-loop configuration.
The magnetization of a permalloy film magnetoresistive bridge can be "set" using a coil that has a direction of magnetic field parallel to the magnetization, i.e., "easy axis", of the magnetic field sensor. Depending on the direction of current in the coil, the magnetization and the input axis of the magnetoresistive bridge can be changed by an angle of 180.degree.. Related art has suggested that switching the magnetization of a magnetoresistive bridge with a coil back and forth from 0.degree. to 180.degree. and so on, and then reading the bridge in each direction and then differencing the readings, the sensor output will be insensitive to thermal drifts and to "unsetting" the bridge in large magnetic fields. This suggested approach was made only in the context of an open-loop configuration which does not eliminate a temperature-dependent offset in the output since the four magnetoresistors of the bridge are not matched well enough to cancel out the total imbalance of the bridge.